Op Amps

[puck123]

Been struggling with this for a while now.

Trying to figure out Rf and Ra on a op amp circuit but unsure what equation to use, I have values for v1 v2 v3 and vout any help would be appreciated

 

[Harald Kapp]

Welcome to EP.
A schematic diagram with the components, nets and values properly labeled could be of some help in understanding and answering your question. Don't you think so, too?

 

[puck123]

 

[hevans1944]

Since this is homework, we aren't going to solve your problems for you, but here are a few tips to get you started...
First, assume the op-amp has infinite gain. This means that for a finite output V_o = 5V that V_p-V_n = 0; that there is no current flowing between the inverting (-) and non-inverting (+) op-amp inputs; and the same current flows through R_f as flows through R_a.

 

[Harald Kapp]

One more tip: calculate VP from V1, V2 and V3 assuming ideal voltage sources (R_i=0 and an ideal OpAmp (input resistance = infinite equiv. to no input current as stated by Hop).

 

[puck123]

Thankyou, I was under the impression I could work out the gain = Vout/ Vin

 

[hevans1944]

Thankyou, I was under the impression I could work out the gain = Vout/ Vin

If this were the usual op-amp circuit, with non-inverting gain for a single input applied to the (+) input, then you could say that the "gain" for that input is 1 + R_f / R_a. You should derive this equation yourself to prove you know what you are doing.

However, it becomes a little more complicated when the output voltage and the input voltages are all specified but the resistor values are not. Do what @Harald Kapp said: calculate the voltage at V_p. This will be the same as the voltage at V_n. From this information you should be able to calculate resistor values for R, R_f and R_a to "make it so" to produce 5V output with the three specified input voltages. Note that the resistor values are arbitrary in their absolute values.